feat: Add referrer controls

[RKNF404]

These patches do the following:

• adds a new feature to disable referrers across origins
• exposes more referrer related features via about:flags
• enables cross-origin referrer trimming by default

~Drafting because this is very experimental.~

[RKNF404]

All feedback welcome

[qoijjj]

Thanks, I'll take a look sometime this weekend